The present invention relates to air compressors, in particular, oilless air compressors.
Generally, an oilless air compressor (also termed an air pump) provides a supply of compressed air. One configuration of an oilless air compressor includes an electric motor rotating an eccentric which, in turn, causes a piston to reciprocate up and down within a cylinder. The eccentric translates the rotary motion of the motor into a reciprocating motion for the piston. On a piston down-stroke air is pulled into the cylinder and on a piston up-stroke air is pushed out of the cylinder.
In such a design, a valve plate closes the end of the cylinder above the piston. The valve plate includes one or more inlet valves that allow air at atmospheric pressure to be pulled into the cylinder on the piston""s down-stroke, but do not allow compressed air to escape to the atmosphere on the piston""s up-stroke. The valve plate also includes one or more exhaust valves that allow compressed air to be pushed out of the cylinder on the piston""s up-stroke but do not allow the compressed air to be pulled into the cylinder on the piston""s down-stroke.
A valve plate in such an arrangement may include an outlet port leading to, for example, a pressurized air tank. On a piston up-stroke, air flows out of the outlet valves, into a chamber above the valve plate, and out of the outlet port. The arrangement of the outlet valves and the outlet port may be such that the output air flow effectively makes a 180 degree turn in the chamber, rising straight up out of the cylinder, being redirected, and exiting the outlet port in the opposite direction. This change in airflow creates turbulence and back pressure, lowering the efficiency of the compressor.
The outlet port in such a valve plate may be formed from both a threaded portion and a compression fitting made, for example, of brass. The compression fitting includes two threaded portions: one connecting with the threaded portion of the valve plate and another connecting with an outlet tube, which may be made of metal such as copper or aluminum. The tube attaches to the fitting via a compression nut and sleeve. The fitting is a discrete component which results in an increased parts count, a potential leak point, a more complex manufacturing process and greater costs.
One eccentric as used in such a compressor design includes a bearing boss, which lies outside of the axis of the motor. As the motor turns the eccentric boss moves in a circle. The boss is rotatably attached to the bottom of the piston by rotatably connecting to a piston bearing bore, a circular hole at the bottom of the piston. As the motor turns the eccentric, the piston is moved up and down. The eccentric boss is surrounded by a bearing; the bore at the bottom of the piston clamps around the bearing. The bearing reduces friction between the eccentric boss and the piston. The piston bearing bore may not be a complete circle in that a slit or small gap exists at the bottom of the piston. This slit or gap allows the bore to expand and contract slightly and allows the tension of the piston bearing bore against the bearing to be adjusted. Two clamping structures extend from the bottom of the piston, one on either side of the slit or gap, and include screw holes. A screw and bolt may be inserted into the clamping structures to alter the tension of the bore against the bearing.
The piston may be die-cast as one component. As the die-cast tool closes, molten metal is injected into the tool, and then the tool separates. The screw hole through the clamping structures extends in the direction that the tool parts separate; to create the hole a core pull is added to the die-cast tool. The use of the core pull adds to the cost of creating the piston.
Such a compressor configuration typically includes a pump frame which is attached to the motor assembly and also to the cylinder. The pump frame supports the cylinder and, since the axle of the motor extends through a bore in the pump frame to attach to the eccentric, the pump frame also helps to support the piston. A bore in the frame holds a bearing which supports the motor axle. The frame bore may include a lip on its outside edge which provides a stop for the bearing when it is pressed into the bore during manufacturing. Such a lip increases the distance between the portion of the frame supporting the axle (through the bearing) and the piston, thereby increasing the moment of force and thus increasing the stress in the frame bearing and the frame.
The spacing of the cylinder outwardly from the motor also adds to the moment and the stress on the bearing. Further, a compressor may include a pump frame with a face which is bowed outward, away from the motor. This also increases the distance between the portion of the pump frame supporting the axle and the piston.
In certain compressor designs, as the piston is forced up and down by the eccentric, the piston also wobbles, or rocks within the cylinder. Such designs may include a flexible seal, formed of a material not requiring oil lubrication, which extends around the perimeter of the piston to ensure the space above the piston is sealed as the piston rocks.
One factor reducing the life of such a piston seal is the angle of the piston during the compression stroke. When the piston is at its top dead center, the head of the piston is flat with respect to the cylinder and the surface of the cylinder head is perpendicular to the axis of the cylinder. Due to piston wobble, however, the piston head is slanted against the cylinder during both the up-stroke and the down-stroke. During the up-stroke, in which air is compressed, the piston seal is pressed unevenly against the cylinder, causing excessive wear of the piston seal.
As air is compressed by the piston, it is heated. The heated air heats components of the air compressor, causing faster wear and reducing operating efficiency. An important factor contributing to piston seal wear is its operating temperature; as the operating temperature increases the life of the seal decreases. To reduce the temperature of such pumps a cooling fan may be included. Due to the location of the fan and the arrangement of the components of certain compressors, such a cooling fan may blow air in a direction more or less perpendicular to the axis of the cylinder. Such an air flow arrangement, however, cools the cylinder inefficiently. The fan is typically connected directly to the eccentric boss and thus rotates at the same speed as the motor. While the compressor and motor may operate at different speeds, the fan may be most efficient at only one speed.
One technique for reducing heat in air compressors is described in U.S. Pat. No. 5,937,736 to Charpie. Charpie describes a piston cap having cooling fins. The piston cap is secured to the piston head, and the cooling fins extend through holes on the piston head. Such a solution is imperfect, as heat is effectively removed only from the piston head. While the cap is secured to the head, heat does not transfer effectively across gaps in metal, and thus the piston head and the piston rod (which is integral with the piston head) are not cooled by the heat sink action of the fins. The size, shape and number of the holes in the piston head limit the size, shape and number of the cooling fins. Further, a piston head with holes for cooling fins may be harder or more costly to manufacture. It is desirable to have a more efficient means for cooling a piston that also allows for easier and less costly construction.
As discussed, when the piston is at its top dead center, the head of the piston is flat with respect to the cylinder, and the surface of the cylinder head is perpendicular to the axis of the cylinder. As the piston tilts away from top dead center, one edge of the piston head rises higher than the center of the piston head. Thus a clearance volume must be provided between the top of the piston and the valve plate. This clearance volume results in a dead space above the piston, reducing the efficiency of the compressor and increasing the heat levels in the compressor.
It is desirable to have an air compressor which overcomes at least some or all of the aforementioned shortcomings of known air compressors.
The present invention provides an air compressor which overcomes the above-described problems of known air compressor designs. An air compressor in accordance with the present invention is more efficient in operation; comprises a piston which experiences less wear, has a more efficient means of cooling and is less expensive to manufacture; allows for reduced stress on the frame and bearings in operation; comprises a more efficient and effective cooling system; and is easier and less costly to construct.
An air compressor according to a preferred embodiment of the present invention includes a valve plate with cooling fins and a piston also having cooling fins. The valve plate includes an integral and angled valve plate outlet suitable for direct connection to a tube. Making the valve plate outlet integral with the valve plate allows for a simpler valve plate with a reduced number of parts, and thus a lesser cost. Angling the valve plate outlet relative to the valve plate reduces turbulence in the space above the cylinder as air is expelled from the compressor, as the exiting air is redirected at an angle of less than 180 degrees. This helps to lower flow resistance which decreases back pressure and increases efficiency.
In an exemplary embodiment, the bearing bore of the pump frame is lipless, decreasing the distance between the portion of the frame supporting the axle and the piston, decreasing the moment of force along the axle and the piston, and thus decreasing the stress on the frame and the frame bearing. Because the pump frame is flat and a portion of the cylinder is over a portion of the motor, the distance between the piston and the portion of the frame supporting the axle is decreased.
In a preferred embodiment, the piston of the compressor includes a novel design allowing for a less expensive casting process. The tension of the piston bearing bore may be adjusted by applying tension to two clamping structures. Tension is provided by a screw passing through clamping screw holes on each of the clamping structures. Each screw hole is formed from a series of structures, such as arcs, arches or barrel portions, which individually do not form the complete circumference of a hole, but when taken together form one or more holes. Such a screw hole does not require a core pull on casting, lowering manufacturing costs.
To improve the longevity of the piston seal, an exemplary embodiment of a piston in accordance with the present invention includes a piston seal which is angled with respect to the major axis of the piston. By thus angling the piston seal, the angle of the piston seal with respect to the axis of the cylinder is closer to perpendicular during a longer portion of the up-stroke than is achievable if the piston seal were not angled with respect to the major axis of the piston.
In a further exemplary embodiment, the piston head has a beveled face formed by two substantially planar portions which meet along a ridge which is substantially perpendicular to the plane in which the piston rocks. As the piston approaches and leaves top dead center, the beveled face of the piston allows the piston to approach the valve plate more closely, thereby reducing efficiency-robbing dead space between the piston and the valve plate.
In a preferred embodiment, the piston also includes cooling fins arranged on the back of the piston head and on the connecting rod. In addition to cooling the piston head directly, the cooling fins also cool the connecting rod which provides a large cooling area, substantially larger than the area of the piston head. The connecting rod is cooled by the ambient air through convection. The connecting rod, in turn, cools the piston head by conduction. This arrangement provides superior cooling of the piston over known arrangements.
In a preferred embodiment, the compressor includes a fan which operates efficiently at different speed settings. The fan is a radial fan including two sets of fan blades, a set of inner flat fan blades which operate most efficiently at a first range of fan speeds and a set of outer curved fan blades which operate most efficiently at a second range of speeds. The fan blows air through the compressor in a novel air cooling pattern, propelling air axially upward along the outside of the cylinder, increasing cooling efficiency.